EFFECT OF SMOKING CESSATION ON PULMONARY CARBON-MONOXIDE DIFFUSING-CAPACITY AND CAPILLARY BLOOD-VOLUME

We studied 16 smokers, with a mean age of 41 yr (SD, 12 yr) and a mean DL(CO) of 81% predicted (SD, 19%), before and after smoking cessation. Two subjects were able to stop smoking for only 24 h, whereas 14 subjects abstained for 1 wk, 11 for 1 month, and five for 3 months. The initial mean DL(CO) in ml/min/mm Hg was 18.9 (SD, 4.6) after correction for COHb back pressure and the reduction in hemoglobin because of COHb. A week after smoking cessation there was a significant increase in DL(CO), to 20.8 (SD, 5.4), p = 0.001. There was no further increase in DL(CO) at 1 month or at 3 months. In four subjects tested while they were still smoking and 24 h after smoking cessation, there was a significant increase in DL(CO) after correction for COHb, from 17.4 (SD, 1.5) to 19.8 (SD, 1.3), p = 0.02. These results indicate that after smoking cessation there is a rapid improvement in DL(CO), suggesting that smoking had previously decreased DL(CO). However, there may also be an irreversible component to the reduction of DL(CO) In some of the subjects in whom the DL(CO) remained abnormal even after continued smoking cessation for 1 month.